Hydrogenation of crystalline silicon involves the bonding of hydrogen atoms to crystallographic defects or contamination within the silicon lattice in a way that prevents that defect or contaminant from acting as a recombination site for minority carriers. This is known as passivation of the particular recombination site. This is important for semiconductor devices that require long minority carrier lifetimes such as solar cells and particularly where cheap silicon is used that often has poor crystallographic quality and/or purity and therefore needs passivation to bring the quality to acceptable levels for high efficiency solar cells.
Low cost silicon in general has much higher densities of silicon crystallographic defects and/or unwanted impurities. These lower the minority carrier lifetime of the silicon and therefore reduce the efficiencies of solar cells made from such material. Passivation of such defects and contaminants to improve minority carrier lifetimes is therefore an important part of being able to fabricate high efficiency solar cells when using lower quality silicon than that routinely used by the microelectronics industry such as with floatzone (FZ) wafers formed from semiconductor grade silicon.
Failure of Existing Commercial Solar Cells to Capitalise on the Potential of Hydrogen Passivation
Currently, without a full understanding of the hydrogenation process and its potential, the designs of commercially manufactured solar cell structures are not ideal to facilitate hydrogenation throughout the cell, and this is reflected in the poor bulk lifetimes for technologies using standard commercial grade p-type wafers.
The ability of hydrogen to move throughout silicon is greatly inhibited by interactions with dopant atoms. For example, in equilibrium in n-type silicon, almost all hydrogen is in the negative charge state and in p-type silicon almost all hydrogen is in the positive charge state. However these states in the respective polarity of silicon can lead to the neutralization of dopant atoms, and can no longer move throughout the silicon. This behaviour of hydrogen in silicon has not been understood or has been overlooked in the past, with the result that attempts at hydrogenation have been less effective than would have been believed by cell designers.
For example, H+ can interact with ionised boron atoms (B−) to form neutral boron-hydrogen (BH) complexes. Similarly, H− can interact with ionised phosphorus atoms (P+) to form neutral phosphorus-hydrogen (PH) complexes.
The dissociation of the dopant-hydrogen complexes is difficult, as even if there is sufficient thermal energy to dissociate the complex, the Coulombic attraction between the dopant atom and the atomic hydrogen (H− for phosphorus and H+ for boron) prevents the escape of the hydrogen atom, and a rapid reformation of the dopant-hydrogen complex is likely.
It can now be seen that the main reasons for poor hydrogenation in the past include: heavy doping in emitters blocking hydrogen from penetrating deep within the silicon; absence of hydrogen sources from one or both surfaces; aluminium alloyed regions acting as sinks; failure to achieve the right charge state for the atoms of hydrogen to facilitate their bonding to certain types of defects and impurities; and no means of trapping of the hydrogen.
Conventional screen Printed Solar Cells dominate commercial manufacture however these have many features that limit the ability to hydrogenate the silicon properly. Firstly, there is a hydrogen source on only one side of the wafer. Since this hydrogen source is generally located at the front surface, in the form of a SiOxNy—H, or SiNx—Hy dielectric, the hydrogen being released into the silicon, struggles to make it deep into the bulk of the wafer due to the heavily doped region within the emitter.
Another limitation is due to the metal-silicon interfaces, which are largely unshielded and so act as sinks that remove the hydrogen ions. Once the hydrogen ions are within the sink region, sites such as di-hydride bonds annihilate the hydrogen ions, forming stable hydrogen molecules that cannot then bond with the silicon to passivate defects. This effect is particularly strong at the Aluminium contact at the rear surface, which is common to almost all commercially manufactured cells. During the firing of the contacts, the molten aluminium alloy is directly against the non-diffused silicon, thereby providing no means of blocking the hydrogen so that the molten region acts as a sink which removes much of the hydrogen.
The latest screen printed cells, which use a selective emitter, overcome some of these issues by using a predominantly lightly doped emitter that allows hydrogen to enter the bulk more easily and also has heavier doping under the metal that helps isolate the metal-silicon interface. However they still suffer from the limitations associated with the aluminium alloy and no rear hydrogen source, plus the peak doping in the emitter is still well above the preferred levels for allowing easy penetration of the hydrogen atoms. Furthermore, even if the concentration of hydrogen atoms reached suitable levels for good passivation of the silicon material, there is no attempt to either generate the preferred charge state for the hydrogen to enhance its bonding ability to certain defects or any attempt to prevent the reactivation of recombination sites during the cool-down that follows thermal processes above 400° C.
Similarly, Pluto cells which use technology having similar attributes to Laser Doped Selective Emitter (LDSE) technology have local heavy doping under the front contacts that help to isolate the hydrogen from the metal-silicon interface. Pluto cells also have a lightly doped emitter that makes it easier for hydrogen to penetrate from the dielectric hydrogen source on the front surface into the silicon wafer. However the surface concentration of phosphorous in Pluto cells is still too high to be optimal. Pluto cells also do not have a hydrogen source at the rear and suffer from the same issues with the rear contact and molten Aluminium alloy acting as a sink for the hydrogen. Furthermore, even if the concentration of hydrogen atoms reached suitable levels for good passivation of the silicon material, there is no attempt to either generate the preferred charge state for the hydrogen to enhance its bonding ability to certain defects or any attempt to prevent the reactivation of recombination sites during the cool-down that follows thermal processes above 400° C.
Sunpower's commercial cells, with a rear collecting junction, rely on using very high quality wafers that therefore achieve good performance without hydrogenation. However their cell structure and processing are not conducive to hydrogenation of the wafer in any case. Sunpower cells do not attempt to facilitate hydrogen penetration easily into either surface with surface oxide passivating layers acting to at least partially block the hydrogen. In addition, SunPower does not attempt to provide a rear surface hydrogen source, but even if there was one, most of the rear surface is heavily doped which would also prevent hydrogen from entering from a hydrogen source at the rear. In addition, the very high temperature processing on these cells is not conducive to retaining the hydrogen required for hydrogenation. Furthermore, even if the concentration of hydrogen atoms reached suitable levels for good passivation of the silicon material, there is no attempt to either generate the preferred charge state for the hydrogen to enhance its bonding ability to certain defects or any attempt to prevent the reactivation of recombination sites during the cool-down that follows thermal processes above 400° C.
Sanyo's Heterojunction with Intrinsic Thin layer (HIT) cell also uses a wafer with much higher minority carrier lifetime than the standard commercial p-type wafers. However, hydrogenation of the wafers would not be possible in any case since the HIT cell structure is based on having amorphous silicon on both surfaces of the cell; it is widely reported that the temperatures required for hydrogenation would seriously degrade the quality of the amorphous silicon and its passivation of the crystalline silicon surfaces.
Yingli's Panda cell is another commercial cell that is based on high quality n-type wafers that therefore achieve good performance without hydrogenation. But in any case, while little is known about the surface coatings of this new cell and whether it has suitable hydrogen sources in contact with the silicon at the surfaces, the cell has high doping at both surfaces (p+ at the front and n+ at the rear) that would block the hydrogen from getting into the silicon wafer anyway from either surface.
CSG Solar's thin film cell design is a commercial technology that could potentially have hydrogen sources on both surfaces, but the required crystallisation of the amorphous silicon is such a long and high temperature process that it drives out all the hydrogen from the source placed adjacent to the glass surface. This then results in the silicon nitride layer adjacent to the glass and the glass itself to act as sinks for any hydrogen that makes it through from the other side of the silicon. To add to this, the cell structure uses heavily doped surfaces on both the front and back that blocks most of the hydrogen entering the silicon, so most of it never reaches the silicon that needs to be passivated. Furthermore, even if the concentration of hydrogen atoms reached suitable levels for good passivation of the silicon material, there is no attempt to either generate the preferred charge state for the hydrogen to enhance its bonding ability to certain defects or any attempt to prevent the reactivation of recombination sites during the cool-down that follows thermal processes above 400° C.